(1) Field of the Invention
The present invention relates to methods and apparatus for applying a coating solution such as SOG (Spin On Glass, also called a silica coating material), photoresist or polyimide resin to substrates such as semiconductor wafers, glass substrates for photomasks, glass substrates for liquid crystal displays or glass substrates for optical disks (hereinafter referred to simply as substrates or as wafers). More particularly, the invention relates to a technique of supplying a coating solution to the surface of each substrate to form a coating film in a desired thickness thereon.
(2) Description of the Related Art
In a conventional coating solution applying method of the type noted above, a coating solution supplied to a substrate forms a substantially circular drop (hereinafter referred to as a core) on the substrate. The drop increases in diameter for a time. Subsequently, the coating solution begins to flow in a plurality of rivulets (hereinafter referred to as fingers) extending radially from the core toward the edge of the substrate. After the fingers reach the edge of the substrate, a large quantity of the coating solution supplied to the core flows through the fingers to scatter to the ambient. Thus, a long time and a large quantity of the coating solution are consumed before the entire substrate surface is covered by the coating solution. In order to cover the entire substrate surface, it is necessary to take into account the quantity of coating solution scattering to the ambient, and to supply the coating solution in a correspondingly increased quantity.
To overcome the above disadvantage, a method has been proposed in which, by way of pretreatment, a solvent is dripped onto a substrate to spin-coat the entire surface thereof first. This step is taken to facilitate spreading of the coating solution over the substrate surface. Then, the coating solution is dripped onto the substrate to spin-coat the surface thereof.
The conventional method noted above has the following drawbacks.
The solvent used in the pretreatment tends to stagnate in recessed parts of a circuit pattern formed on the substrate. The solvent trapped under a coating film formed could turn into bubbles when the film is baked. This results in irregularities of the coating film obtained ultimately.
In addition, while the pretreatment with the solvent facilitates spreading of the coating solution, thereby reducing consumption of the latter, the solvent per se has to be consumed in a large quantity.
The present invention has been made having regard to the state of the art noted above, and its object is to provide a coating solution applying method in which a solvent is sprayed in advance of supplying a coating solution, to avoid irregularities due to the solvent, and to drastically reduce the quantity of coating solution needed to form a coating film while suppressing solvent consumption.
The above object is fulfilled, according to this invention, by a method of applying a coating solution to a surface of a substrate to form a coating film of desired thickness thereon, comprising the steps of:
(a) spraying a solvent over the surface of the substrate maintained still or spun at a first rotational frequency;
(b) supplying the coating solution to a region centrally of the surface of the substrate maintained still or spun at a second rotational frequency;
(c) accelerating the substrate to a third rotational frequency before the coating solution supplied at step (b) above spreads and entirely covers the surface of the substrate; and
(d) spinning the substrate at a fourth rotational frequency for a predetermined period of time to adjust thickness of a coating film covering the surface of the substrate.
First, before supplying the coating solution to the surface of the substrate, the solvent is sprayed over the surface of the substrate maintained still or spun at the first rotational frequency (step (a)). The solvent sprayed before application of the coating solution produces the effect of reducing the angle of contact between the coating solution and the surface of the substrate. When the coating solution is supplied to the substrate subsequently, the coating solution may spread extremely smoothly over the substrate surface. Since the solvent is not supplied in droplets but is sprayed in mist, the solvent may cover a large area on the substrate within a short time. Even if recesses such as of a circuit pattern are formed on the surface of the substrate, the solvent hardly stagnates in such portions.
After the solvent is sprayed, the coating solution is supplied to a region centrally of the surface of the substrate maintained still or spun at the second rotational frequency (step (b)). In the initial spreading stage of the coating solution, numerous fingers develop from a circular core of the coating solution, and begin to extend radially toward the edge of the substrate. When the numerous fingers reach the edge of the substrate, the coating solution would flow through the fingers to scatter to the ambient. Thus, a large quantity of coating solution would be wasted. To avoid such a situation, the substrate is accelerated to the third rotational frequency before the coating solution supplied spreads and entirely covers the surface of the substrate (step (c)).
Under this rotational frequency control, the coating solution exhibits a behavior as shown in FIG. 4.
Where, as in the prior art, the second rotational frequency is maintained, the core Ra and fingers Rb shown as a hatched region in FIG. 4 enlarge and extend toward the edge of substrate W, as shown in a two-dot chain line, under the centrifugal force. However, as the spin of the substrate is accelerated to the third rotational frequency, the fingers Rb are subjected to a force of inertia, i.e. a force acting in the direction opposite to the direction of spin. The resultant of centrifugal force and inertia bends the fingers Rb circumferentially,. thereby enlarging widths thereof. The leading ends of fingers Rb extend under the centrifugal force toward the edge of substrate W (as shown in a dotted line in FIG. 4). The diameter of core Ra also increases. Moreover, since the solvent has been sprayed before the coating solution is applied, the fingers Rb are readily bent to great degrees circumferentially. Though not shown in the schematic view, with the solvent applied beforehand, the fingers Rb are formed more thinly and in a greater number than in the prior art. The diameter of core Ra also increases more quickly than in the prior art.
Thus, the fingers Rb not only extend toward the edge of substrate W, but greatly increase in width in a circumferential direction. The regions between the fingers Rb diminish rapidly, to shorten the time taken for the coating solution to cover the entire surface of the substrate. The shortened coating time means that only a short time is taken from start of the supply of the coating solution to finish of the supply after the coating solution covers the entire surface of substrate W. In other words, a reduced time is taken from arrival of fingers Rb at the edge of substrate W to finish of the coating solution supply. Thus, a correspondingly reduced quantity of the coating solution scatters to the ambient through the fingers Rb. Thereafter the fourth rotational frequency is maintained for a predetermined period (step (d)) to dispel a superfluous part of the coating solution. In this way, a reduced quantity of coating solution is required to form a coating film of desired thickness.
By accelerating the substrate before the coating solution covers the entire surface thereof, a force of inertia may be applied to the rivulets of coating solution extending radially from the circular drop of coating solution, thereby rapidly diminishing the regions between the radially extending rivulets of coating solution. In addition, since the solvent has been sprayed before the coating solution is applied, thin rivulets of coating solution may be formed in an increased number. The solvent also promotes the concentric increase in diameter of the coating solution, and facilitates circumferential bending of the rivulets under the force of inertia.
Thus, a reduce time is taken for the coating solution to cover the entire surface of the substrate. As a result, a reduced quantity of coating solution scatters to the ambient through the radially extending rivulets. A reduced quantity of coating solution is required to form a coating film of desire thickness. Since the solvent is sprayed before the coating solution is supplied, the solvent can cover a wide range in a short time. Even if recesses of a circuit pattern have been formed on the surface of the substrate, the solvent hardly stagnates in such portions. The coating film formed is free from irregularities due to the solvent. While checking consumption of the solvent as above, the expensive coating solution may be used in a reduced quantity. With the reduced consumption of the solvent and coating solution, semiconductor devices and the like may be manufactured at low cost and with improved throughput.
In a known coating solution applying method, the substrate is spun at high speed when the coating solution is supplied, and thereafter the substrate is decelerated to adjust film thickness, thereby reducing the quantity of coating solution used. In this case, the solvent sprayed would vaporize to diminish the advantage of its use. According to the present invention, the solvent does not vaporize easily, thereby taking full advantage of its use.
Preferably, step (b) of the present invention is executed to start and finish supplying the coating solution while the substrate is spun at the second rotational frequency.
This is what is known as the xe2x80x9cdynamic methodxe2x80x9d in which the coating solution begins to be supplied while the substrate is spun at the second rotational frequency, and the supply is stopped in this state. In this case also, the same effect is produced to shorten the required coating time by accelerating the substrate to the third rotational frequency before the coating solution covers the entire surface of the substrate. Consequently, a reduced quantity of coating solution scatters to the ambient.
It is also preferred that step (b) above is executed to start and finish supplying the coating solution while the substrate is maintained still.
This is what is known as the xe2x80x9cstatic methodxe2x80x9d in which the coating solution begins to be supplied while the substrate is maintained still, and the supply is stopped in this state. In this case also, the same effect is produced to shorten the required coating time by spinning the substrate and accelerating the spin to the third rotational frequency before the coating solution covers the entire surface of the substrate. Consequently, a reduced quantity of coating solution scatters to the ambient.
Further, step (b) above may be executed to start supplying the coating solution while the substrate is maintained still. and finish supplying the coating solution after the substrate begins to be spun toward the second rotational frequency.
This may be said a combination of the static method and dynamic method in which the coating solution begins to be supplied while the substrate is maintained still, and the supply is stopped after the substrate begins to be spun toward the second rotational frequency (this supplying method being referred to hereinafter as the xe2x80x9cstamic methodxe2x80x9d). In this case also, the same effect is produced to shorten the required coating time by accelerating the substrate to the third rotational frequency before the coating solution covers the entire surface of the substrate. Consequently, a reduced quantity of coating solution scatters to the ambient.
In another aspect of the invention, there is provided a method of applying a coating solution to a surface of a substrate to form a coating film of desired thickness thereon, comprising the steps of:
(a) spraying a solvent over the surface of the substrate maintained still or spun at a first rotational frequency;
(b) supplying the coating solution to a region centrally of the surface of the substrate maintained still or spun at a second rotational frequency;
(c) decelerating the substrate to a third rotational frequency lower than the second rotational frequency before the coating solution supplied at step (b) above spreads and entirely covers the surface of the substrate;
(d) accelerating the substrate to a fourth rotational frequency before the coating solution supplied at step (b) above spreads and entirely covers the surface of the substrate; and
(e) spinning the substrate at a fifth rotational frequency for a predetermined period of time to adjust thickness of a coating film covering the surface of the substrate;
wherein step (c) is executed with the coating solution continuing to be supplied at least from start of the deceleration to attainment of the third rotational frequency.
First, before supplying the coating solution to the surface of the substrate, the solvent is sprayed over the surface of the substrate maintained still or spun at the first rotational frequency (step (a)). The solvent sprayed before application of the coating solution produces the effect of reducing the angle of contact between the coating solution and the surface of the substrate. Thus, the coating solution may spread extremely smoothly over the substrate surface. Since the solvent is sprayed in mist, the solvent may cover a large area on the substrate within a short time. Even if a circuit pattern is formed on the surface of the substrate, the solvent hardly stagnates in such pattern portions.
After the solvent is sprayed, the coating solution is supplied to a region centrally of the surface of the substrate maintained still or spun at the second rotational frequency (step (b)). As the coating solution is spread on the substrate spinning at the second rotational frequency, numerous fingers develop from a circular core of the coating solution, and begin to extend radially toward the edge of the substrate as noted hereinbefore. When the numerous fingers reach the edge of the substrate, the coating solution would flow through the fingers to scatter to the ambient. Thus, a large quantity of coating solution would be wasted.
To avoid such a situation, the substrate is temporarily decelerated to the third rotational frequency lower than the second rotational frequency before the coating solution supplied spreads and entirely covers the surface of the substrate (step (c)). This third rotational frequency includes zero rotational frequency, i.e. a state in which the substrate is maintained still. With this rotational frequency control, the coating solution exhibits a behavior as schematically shown in FIGS. 7 through 10. FIGS. 7 and 8 are side views schematically showing the substrate and coating solution. FIGS. 9 and 10 are plan views schematically showing the substrate and coating solution.
When the spin of the substrate begins to be decelerated to the third rotational frequency lower than the second rotational frequency, the enlargement of core Ra and extension of fingers Rb begin to retard. When the third rotational frequency is attained, the growth of core Ra and fingers Rb is stopped substantially and temporarily, compared with the state before commencement of the deceleration. The supply of the photoresist solution is continued at least until attainment of the third rotational frequency. Consequently, the core Ra has an increased quantity of photoresist solution R (FIG. 8) compared with the core Ra before the deceleration (FIG. 7). With the core Ra having the increased quantity of coating solution R, i.e. with the core Ra having increased momentum for growth, and before the coating solution covers the entire surface of substrate W, the substrate is spun again with the rotational frequency increased to the fourth rotational frequency higher than the third rotational frequency (step (d)). Then, the coating solution exhibits a behavior as shown in FIGS. 9 and 10.
If the rotational frequency is maintained as in the prior art, the core Ra and fingers Rb, from the state shown in hatches in FIG. 9, will grow and extend under centrifugal force straight toward the edge of substrate W as shown in a two-dot chain line. Besides, new radial rivulets (hereinafter referred to as new fingers Rbxe2x80x2) develop from the core R having increased in volume. These new fingers Rbxe2x80x2 begin to extend from between the numerous fingers Rb toward the edge of substrate W.
As the rotational frequency is increased from the third rotational frequency to the fourth rotational frequency, the fingers Rb and new fingers Rbxe2x80x2 developing as shown in FIG. 9 are subjected to a force of inertia, i.e. a force acting in the direction opposite to the direction of spin. The resultant of centrifugal force and inertia bends the fingers Rb and new fingers Rbxe2x80x2 circumferentially, thereby enlarging widths thereof, as shown a dotted line in FIG. 10. The leading ends of fingers Rb and new fingers Rbxe2x80x2 extend under the centrifugal force toward the edge of wafer W. The core Ra also increases in diameter. Moreover, since the solvent has been sprayed before application of the coating solution, the fingers Rb and new fingers Rbxe2x80x2 are readily bent to great degrees circumferentially. Though not shown in the schematic views, with the solvent applied beforehand, the fingers Rb and new fingers Rbxe2x80x2 are formed more thinly and in a greater number than in the prior art. The diameter of core Ra also increases more quickly than in the prior art.
Consequently, as shown in FIG. 10, the fingers Rb and new fingers Rbxe2x80x2 not only extend toward the edge of substrate W, but greatly increase in width in the circumferential direction. Before the fingers Rb reach the edge of substrate W, gaps between the fingers Rb are rapidly narrowed with the aid of new developing fingers Rbxe2x80x2. This drastically reduces the coating time required for the coating solution R to cover the entire surface of substrate W. The shortened coating time means that only a short time is taken from start of the supply of the coating solution to finish of the supply after the coating solution covers the entire surface of substrate W. In other words, a correspondingly reduced quantity of coating solution scatters to the ambient through the fingers Rb (and new fingers Rbxe2x80x2). Thereafter the fifth rotational frequency is maintained for a predetermined period (step (e)) to dispel a superfluous part of the coating solution. In this way, a reduced quantity of coating solution is required to form a coating film of desired thickness.
Thus, before the coating solution supplied to the surface of the substrate covers the entire substrate surface, the substrate is temporarily decelerated to the third rotational frequency while the supply of coating solution is continued. Only the coating solution thereby increases in concentric form to gain increased momentum for growth. Subsequently, the substrate is accelerated whereby new flows of the coating solution develop between the radial flows of the coating solution extending from the coating solution in concentric form, with a force of inertia applied to each flow of the coating solution. The gaps between the radially extending flows are thereby narrowed rapidly. In addition, since the solvent has been sprayed before the coating solution is applied, thin rivulets of coating solution may be formed in an increased number. The solvent also promotes the concentric increase in diameter of the coating solution, and facilitates circumferential bending of the rivulets and new rivulets under the force of inertia.
Thus, a reduce time is taken for the coating solution to cover the entire surface of the substrate. As a result, a drastically reduced quantity of coating solution scatters to the ambient through the radially extending rivulets. A reduced quantity of coating solution is required to form a coating film of desire thickness. The solvent sprayed can cover a wide range in a short time, and hardly stagnates in recessed pattern portions formed on the surface of the substrate. The coating film formed is free from irregularities due to the solvent. While checking consumption of the solvent as above, the expensive coating solution may be used in a reduced quantity. With the reduced consumption of the solvent and coating solution, semiconductor devices and the like may be manufactured at low cost and with improved throughput.
In a known coating solution applying method, the substrate is spun at high speed when the coating solution is supplied, and at low speed when adjusting film thickness, thereby reducing the quantity of coating solution used. In this case, the solvent sprayed would vaporize to diminish the advantage of its use. According to the invention defined in claim 5, the solvent does not vaporize easily, thereby taking full advantage of its use.